I want to start off by saying that I am not a highline expert, I have set up a few highlines, but there are others more qualified to comment on highline rigging. This thread is non-exhaustive and was written to express the data generated from my experiment.

In the world of highlining, slackliners normally use two separate systems to protect them should they fall. The first system is a tensioned mainline that they walk across, basically just a slackine. The second is a backup line should the mainline fail. One purpose of this thread is to focus on one specific type of back-up, threaded lines. According to SlacklineExpress.com,

There are differing schools of thought on the fine points of all rigging, and how to make a backup main line is no different. Some people like to thread 11/16" webbing through the 1" webbing on the main line to create a Threaded Mainline and that's it, calling the 11/16" the backup system. Personally, I disagree with this practice. While a threaded main line will get tighter easier for some tensioning systems, it does not increase the strength of your system. From our testing, threaded lines are less than ideal because your backup is pre-tensioned and strength wise the core breaks independently and then the 1" breaks at standard breaking load. Since it doesn't yield any higher tensile strength, nor does it share the load if you ever managed to break your 1".

The purpose of this study is to determine three things. What is the strongest way to terminate a piece of webbing? Is SlacklineExpress.com right? Does threading a mainline have no effect on the ultimate strength of the line? And lastly, how much force does a highline fall actually produce?

Chapter one  Termination of the webbing

I am going to test five ways to terminate webbing, a clove hitch, an overhand knot, a frost knot, a line locker, and a sewn loop. The frost knot is a variation of the overhand knot that introduces a second piece of webbing, which is supposed to reduce the strength loss from tying a knot.

First, we will start with some basic theory. The two most popular ways to terminate a highline or slackline is likely a line locker (or webbing locker) and an overhand knot. So I picked up some cheap pink webbing from the hardware store to test the strength differences between a line locker and an overhand knot.

Overhand knot: 4.55kN Line locker: 5.54kN

I only tested one sample of each, but the initial results look promising, the line locker yielded a 17.7% higher strength than the overhand knot. Moving on, it is time to test the strength of the different types of termination options. Here are the specifications of the webbing I used:

- ABC 1 nylon tubular webbing - Manufacturer rated strength: 17.78kN

I only tested two samples for each termination point. However, the test results were very repeatable. Below are the results of the test. The percentage value expressed in the parentheses is the average offset from the manufacturer rated strength of 17.78kN. In all tests, the material failed at the termination point (the knot, bar tack end, or line locker).

As you can see, the only two termination methods that preserved the webbings rated strength were the line locker and sewn loop, with the sewn loop being the strongest of them all.

Chapter two  Analysis of threaded lines

In this section I am going to visit the theory behind whether threading a line has no effect on the lines ultimate strength, as stated by SlacklineExpress.com, or whether it does increase the ultimate strength of the webbing. SlacklineExpress.com said From our testing, threaded lines are less than ideal because your backup is pre-tensioned and strength wise the core breaks independently and then the 1" breaks at standard breaking load. Before jumping head first, I would like to determine whether the elongation of a narrower piece of webbing is different from the elongation of a wider piece. I want to start with a 3/4" mainline threaded with 1/2 webbing rather than going straight to a 1 threaded mainline.

My theory is that if the elongation of the different pieces of webbing is the same, they should share the load equally up until the breaking strength of one piece of webbing is reached, which would result in its failure, shifting 100% of the load to the remaining piece, which would then result in the immediate failure of the remaining piece. In other words, my theory is that if both the 3/4 mainline and the 1/2 thread stretched equally, the strength of the line should be twice that of the weaker piece (normally the 3/4 piece). If the elongation is different, one piece will see a greater load than the other, which greatly complicates things. I believe that if the elongation of the 1/2 is lower than that of the 3/4, the 1/2 will fail prematurely shifting 100% of the load to the 3/4. Likewise, if the 3/4 stretches less, it will fail first shifting all of the load to the remaining 1/2 piece. But enough guessing, let us just test it.

First, we need to determine how strong the 1/2 and 3/4 samples I am using are. I tested a few samples and came up with the average below. I tested the samples with line locker terminations.

1/2 tubular nylon: 4.95kN 3/4 tubular nylon: 13.8kN

Next we need to determine what the elongation of the material is. This process is a bit tricky because I cannot pull test a long sample. I am limited to a webbing sample length of about three feet so the accuracy of this test will be limited to some extent. I conducted two different tests: the average elongation of the samples that encompasses the stretch from 60 lbs. to material failure (ultimate elongation) and the average elongation of the samples from 60 lbs. to 300 lbs.

So as you can see, the two pieces of webbing do have different elongation parameters. So far this theory is looking in favor what SlacklineExpress.com stated. Although the 1/2 webbing does elongate a bit more than the 3/4 does, the difference between the strength of the 1/2 and 3/4" is greater than the difference between the elongation of the 1/2" and 3/4" samples, meaning it looks like achieving a much higher ultimate strength with a threaded line is looking pretty unlikely.

So finally we are there, time to find out for sure if threading a line increases its strength. I tested the threaded webbing the same way I tested the 1/2" and 3/4 samples, with line locker terminations. The percentage values in the parentheses are the offset values from the 3/4" non-threaded sample.

So a 7% increase - this study is looking in favor of SlacklineExpress.coms theory so far, but we still have a bit more testing to do.

In this next section we will examine 1 webbing threaded with 3/4 webbing. Both the 1 and 3/4 samples are from the same sample pool I have been using in the previous tests. However, before we start testing the breaking strength of threaded 1 we need to examine the elongation of the materials being used in the mainline. Please note that the elongation data presented earlier in this study is only relevant to that section of the study because the method I used to calculate the elongation data is far different from the method I am using below.

Below are some photos showing how I captured the elongation data. Basically, I just pulled on the webbing with a ratchet and recorded the elongation of the webbing every 120 lbs. The graphs below represent the elongation of the webbing alone. The graphs do not include the elongation of any outside sources, such as the attaching carabiners or line lockers.

Figure 7 shows the elongation data of a piece of 3/4 webbing, a piece of 1 webbing, and 1 webbing threaded with 3/4. In figure 7, I tested the samples independently; they were not tested as a single threaded line and analyzed separately. Figure 10 differs from figure 7 in that I tested both the 3/4 and 1 strands together as a threaded line. Basically, figure 7 represents three separate samples and three separate tests in one graph. Figure 10 represents one sample and one test; a test of the threaded line. In figure 10 I measured the length of the threaded line as a single unit, but I measured the load on each strand individually with two load cells. Because I separated the 1 and 3/4 pieces and I terminated them each to a different load cell, it was critical I ensured that each strand was the same length. If they were different lengths, they would elongate differently in relation to each other in a method not analogous with its modulus of elasticity. So rather than trying to measure the length of each strand, I simply placed 80 lbs. of force on the sum of the two stands and adjusted each strand until both load cells read the same weight, that way I knew both strands were precisely the same length.

Here are some photos showing how I captured the load on each strand:

As you can see from the graphs above, the threaded mainline elongated about 4% less at 1400 lbs. than a 1 non-threaded mainline. However, there was only about a 2.5% difference between the 1 and 3/4 at 1400 lbs.

One very interesting feature of Microsoft Excel is its ability to predict future readings in a graph. Because I cannot generate 4,000+ lbs. of force with my ratchet, I am limited to elongation readings within the scope of the ratchet. This is where Excel comes into play. Because we are concerned about the ultimate strength of threaded line, we are also concerned with the ultimate elongation of the webbing. Figure 9 shows a 2nd order polynomial theorized treadline of what the elongation of the webbing would be at higher forces.

Before continuing, we need to take a quick detour. Because I am presenting data that has been theorized by Excel, I want to verify the accuracy of the theoretical treadline created by Excel. Figures 8a and 8b show the accuracy of the treadline in reference to the actual elongation of the 1 webbing. Figure 8a uses a 2nd order polynomial equation to calculate the treadline and figure 8b uses a 3rd order polynomial equation. I separated the three colored lines into separate readings so Excel could not use them all to calculate the treadline. I only used the dataset contained within the blue line to calculate the treadline. As you can see, both treadlines are reasonable accurate within a 250% forecast of the original dataset. Because the 2nd order polynomial seems to be marginally more accurate, I used that order to calculate the treadline in figure 9 instead of the 3rd order.

Please note that these treadlines will only be accurate if the mathematical function of the elongation dataset continues in its current pattern as the material is further strained. If the elongation pattern changes significantly, the treadline will not be accurate. In other words, it is just a theorized treadline generated by a computer.

Next we will examine the most important aspect of this set of tests, the difference in force between the two stands of our threaded 1 sample. Figure 11 shows the difference in force between the 1 and 3/4 strands when they are used together as a threaded line. This graph is very important because it tells us how strong the threaded strand can be. I say can because as we will find later in this chapter, the actual strength of a threaded line is dependent on more than just the strength of the webbing.

Figure 12 shows the theorized difference between the strands at higher forces. I used a linear treadline because the very inconsistent nature of the graph throws any other treadline options off the grid. So the liner treadline is probably of limited accuracy, but it does not matter, what we can take from this portion of the experiment is that there is not a huge difference between the forces experienced by the two strands of a threaded line.

Because we now know the difference in force between the strands, we can predict the breaking strength of the threaded line. We know a single strand of 1 fails at just shy of 4,000 lbs. and a single piece of 3/4 fails at around 3,300 lbs. Excel predicts the difference in force between the two strands at 4,500 lbs. is only about 265 lbs., with the greater load being carried by the less dynamic strand (the 1). Consequently, we can theorize that a 1 threaded line will fail at around 6,600 lbs., which is double the strength of a piece of 3/4.

Lastly I tested the breaking strength of 1 webbing threaded with 3/4. The percentage values in the parentheses are the offset values from the 1" non-threaded sample.

So what happened? We theorized the 1 threaded line would fail at 6,600 lbs., but instead it failed at only 4,750. The answer to this question is under further research and I will update it when an answer is found. So far, experts in the field tell me the increased width of a threaded line is making the line locker termination significantly less efficient and the major loss in efficiency is the cause of the premature failure. That theory makes sense because there are only two possible reasons why the material would fail at a much lower force than what we predicted. First, the difference between the forces carried by each strand widens significantly as the load is further increased (unlikely). The second possibility is that the line locker is indeed less efficient when a threaded strand is passed through it.

CONCLUSION

So it appears that threading 3/4" webbing does not increase the strength by very much, only a few hundred pounds (under further examination). However threading 1 did yield nearly a 21% increase in strength. However, although the strength of the 1 was increased by 21% through threading, that is still only an additional 3.61 kN  nothing to write home about (under further examination). We learned that the load was almost equally shared between the 1 mainline and 3/4 thread. So what very likely happened in the case of the 3/4 threaded mainline was that the inner 1/2 core failed and then the 3/4 piece failed. The 1/2 core only held 1,112 lbs. in my tests, so a doubling of that yields 2,224 lbs. The likely reason the 3/4 threaded line held 3,323 lbs. instead of only 2,224 lbs. is because the difference in elongation between the 1/2 strand and the 3/4 strand was much greater than the difference between the 3/4 strand and 1 strand. Accordingly, the lower elongation strand (3/4 strand) likely held a greater amount of force then the 1/2 strand.

This study is under further research and will be updated within a few weeks.

SECTION 3  Highline fall analysis with a strain gauge indicator

In the next chapter I am going to determine how much force a leased highline fall actually produces and how strong your webbing and anchor actually needs to be.